1. Field of the Invention
The invention relates to a projection objective for wavelengths ≦193 nm with at least six mirrors which are arranged along an optical axis.
2. Description of Related Art
One central problem in the design of EUV lithographic systems is the transmission of the system, both in the area of the illumination as well as the projection system. The transmission is determined by two factors: on the one hand, by the number of the optical elements, e.g. normal-incidence and grazing-incidence mirrors, filters, etc., and on the other hand by the reflectivity of the optical elements. In order to optimize the transmission it is necessary both to minimize the number of the optical elements as well as maximize the reflectivity of every single optical element. Preferably this occurs under consideration of the respective boundary conditions, especially the system specifications concerning the imaging power and the aspects concerning construction and production.
One possibility to reduce the number of the optical elements is to choose the intersection distance or the intercept distance of the entrance pupil of the projection lens system to be negative. This has been described in WO 2004/010224 A2. In the illumination system as known from WO 2004/010224, it is possible to omit normal-incidence mirrors within the illumination system relative to systems with positive intersection distance or intercept distance leading to a double of the transmission of the system. However, first projection systems as shown in this application show a very small working distance on the object side, so that it is not possible or with much difficulty to couple in an illuminating beam. Although the other system shown in WO 2004/01024 A2 has a large working distance on the object side of approximately 400 mm, the incidence angles on the first three mirrors are so large that the reflectivity of these mirrors is comparatively small.
In the projection system as shown in WO 2004/010224 with negative intersection distance or intercept distance the embodiments with small incidence angles have a maximum distance between reticle and second mirror M2 of approximately 226 mm at an overall length of 1500 mm. This corresponds to a value of the free working distance of approximately 15% of the overall length of the projection system. The embodiment with large incidence angles comprises a distance between reticle and mirror of 400 mm.
A system with negative intersection distance is also shown by the specification U.S. Pat. No. 6,781,671 B2. The entrance pupil also has a negative intersection distance in this case too. The incidence angles especially on the mirrors M22, M24 and M25 are so large that the reflection losses on these mirrors are very high. The six-mirror projection objective as shown in U.S. Pat. No. 6,781,671 B2 comprises a working distance on the object side of approximately 29 mm at an overall length of 698 mm, i.e. the working distance is only 4.2% of the overall length of the objective and has a very large incidence angle >20°. The six-mirror system of U.S. Pat. No. 6,781,671 B2 has a distance between reticle and second mirror of 55 mm, i.e. 7.9% of the overall length.
The U.S. Pat. No. 6,033,079 shows a projection system with minimized incidence angles, so that in the case of choosing a respectively adjusted coating the reflectivity of the individual mirrors and thus the transmission of the projection lens system is maximal. However, the systems known from U.S. Pat. No. 6,033,079 have a positive intersection distance of the entrance pupil, so that the illumination system shows very high losses as a result of this positive intersection distance.